1. Field of the Invention
The present invention relates to a method of manufacturing a resin structure reinforced with long fibers. More particularly, the invention relates to a method of manufacturing a resin structure reinforced with long fibers in which impregnation of the fibers with a resin melt is improved by specifying the degree of fiber loosening of a continuous fiber bundle.
2. Description of the Related Art
Elongated resin structures reinforced with fibers in which thermoplastic resins are reinforced with continuous fibers have mechanical properties much superior to those of the structures reinforced with short fibers. Such structures are beneficial because they can be cut and formed Into pellets or similar materials. Due to these advantageous features, they have recently become of particular interest. Elongated thermoplastic resin structures reinforced with fibers are generally manufactured by the so-called pultrusion method by impregnating a thermoplastic resin into a continuous fiber bundle for reinforcement while the bundle is passed through a cross-head extrusion die, after which the resin-impregnated fiber bundle is drawn out though the die (U.S. Pat. No. 3,993,726), or by drawing a continuous fiber bundle for reinforcement through a melt thermoplastic resin to wet the bundle and pulling out the wet bundle through a die (Japanese Patent Application Laid-open (kokai) No. 57-181,852). After undergoing the pultrusion method, the structures are cut to a desired size.
Other types of structures which are obtained by reinforcing thermosetting resins with continuous fibers are manufactured by various methods. For example, in the so-called filament winding method, fibers such as glass roving are coated with a resin before the resin has set and wound around a rotating frame while applying tension, and then the resin is allowed to set. In the preforming method, a semi-set prepreg is formed. In the pultrusion method, profile extrusion is first performed, after which the extruded product is completely set.
In the manufacture of these resin structures reinforced with long fibers, irrespective of whether a thermoplastic resin or a thermosetting resin is used, it is important that a matrix resin be uniformly impregnated into continuous fiber bundles for reinforcement in order to achieve consistent manufacture and to obtain good mechanical properties of the resulting resin structures reinforced with long fibers as well as a good appearance and to prevent fiber filaments from falling off the products.
To this end, it is a general practice that a fiber bundle which has been bound is loosened while passing through tension bars or rolls under tension, and the loosened fiber bundle is impregnated with a molten or liquid-state resin. However, uniform impregnation is difficult to achieve by this method because impregnation is affected by the diameter or the Tex number of the continuous fibers which are used, although the degree of fiber loosening of a continuous fiber bundle is presumed to have some relation to uniform impregnation.
In view of the foregoing, the inventors of the present invention conducted extensive studies of the most suitable degree of loosening of a fiber bundle when a loosened continuous fiber bundle is impregnated with a resin melt, and found it important that the fiber-loosening index be in a specific range, leading to completion of the invention.